The term 'EARTH's Core, Mantle, or Crust' refers to the three distinct layers of the EARTH. The core is the innermost LAYER and is composed of iron and nickel. The mantle is the LAYER between the core and the crust and is made up of hot, solid rock. The crust is the outermost LAYER and is composed of solid rock that is broken into pieces called tectonic plates. The crossword solution for this term is 'LAYER', which accurately describes the three distinct layers of the EARTH.